Air heater fuel control system



June 27, 1961 A. H. RoBsoN AIR HEATER FUEL CONTROL SYSTEM Filed OOb. 30, 1959 United States PatentOce 2,990,117 Patented June 27, 1961 2,990,117 Y AIR HEATER FUEL CONTROL SYSTEM p Aubrey H. Robson, Rock Island, Ill., assignor to American Air `Filter Company, Inc., Louisville, Ky., a corporation of Delaware Filed Oct. 30, 1959, Ser. No. 849,830 8 Claims. (Cl. 236-10) vThis invention relates to a fuel and combustion control system Ifor a portable air heater of the uid yfuel burning type.

This invention has application to that type of heater in which fuel is supplied to a bypass type burner nozzle at a relatively constant pressure, part of the fuel normally being discharged from the nozzle into the burner and the remaining fuel passing through a built-in nozzle bypass to a nozzle return line. The rate at which fuel is discharged into the burner is dependent upon the fuel back-pressure in the nozzle return line-a high backpressure causing more fuel to be discharged into the burner than a low back pressure. The degree of backpressure built up in the nozzle return line may be controlled by a throttling valve controlled in response to variations in temperature of the heated air discharged from the heater. Such -a heater so controlled is exemplified by Hubbard U.S. Patent 2,758,591.

I have recently proposed, as disciosed in my co-pending application Serial No. 849,882, filed October 30, 1959, that where a heater having a bypass type fuel control system is applied to heat a relatively confined space, and in which the air from the space is recirculated or returned to the heater from that space, that a pair of throttle valves arranged in parallel be utilized in the nozzle return line. One valve is controlled in response to the heater discharge air temperature and the other is controlled in response to the heater inlet air temperature. The system is arranged so that during the space warm-up period the discharge air temperature responsive valve eifectively controls burner fire at a rate providing the selected discharge air temperature, and then, after the space reaches a comfort temperature due to sustained operation of the heater at the selected discharge air temperature, the inlet air temperature responsive valve assumes control to provide a reduced rate of burner re ample to satisfy the reduced demand for heat in the space. While such an arrangement provides a rapid warm-up of the space to be conditioned and thereafter provides air at a relatively low discharge temperature, under certain conditions the heat loss of the conditioned space may be so low that even the relatively low temperature air `discharged into the space will result in overheating of the space.

Specifically, assuming a heater having aecapacity of from 20,000 to 200,000 B.tu. per hour, a Ventilating air volume of 350 c.f.m., and a heater inlet air temperature of 70, then, at the minimum capacity of 20,000 B.t.u. per hour the temperature of the air discharged into the van or space will be about 123 F. It will be readily appreciated that if the heat loss of the space is fairly low, the continued addition of extra heat to the confined space can result in overheating of the space.

Therefore, one object of the present invention isto provide, in a fuel control system for a heater of the class described, means to control the burner lire in an on-off fashion in response to a condition indicative of space overheat occurring during continuous tiring at the minimum heating capacity of the heater.

In carrying out the invention, means sensing the fuel back-pressure in the nozzle return line are provided to respond to a relatively low pressure corresponding to a condition of minimum burner fire to cause the ow of fuel to the nozzle to be terminated and shunted around the nozzle through a separate conduit connecting the' nozzle supply line and nozzle return line. The fuel back` pressure continues to be controlled in accordance withl the difference between a selected air temperature and a sensed air temperature so that fuel flow to the nozzle will be re-established in response to a rise in fuel back; pressure indicative of the need of heat. Y

To avoid causing a change of material magnitude in the fuel pressure in the nozzle return line upon actuatingy the means diverting the fuel to the nozzle shunt conduit. I provide flow restricting means in the nozzle shunt conduit to exert approximately the same llow resistance as encountered when ilow is through the nozzle.

The invention will be explained in connection with the accompanying `drawing illustrating several embodiments by way of example, and wherein:

FIGURE 1 is a diagrammatic view of a fuel control system and heater embodying the present invention;

FIGURE 2 is an electrical diagram illustrating electrical connections of elements used where electrical power is kavailable for powering the heater;

FIGURE 3 is a diagrammatic view of a portion of an alternative fuel control system and illustrating in cross section one specific fuel pressure powered valve adapted to divert fuel from the nozzle.

Referring to FIGURE 1, the heater includes: forced air blower means 2 providing ventilating air to be heated, the Ventilating air owing as indicated by the solid arrows; combustion air blower means 4 providing combustion lair for the burner and combustion chamber 6, the combustion air flowing as indicated by the dotted arrows into the combustion chamber 6 and then out of a stack 8; means defining a heat exchange passageway 10; a burner nozzle of the bypass type 12 adapted to receive fuel and discharge at least part of it into the burner 6; ignition electrodes 14 adjacent the nozzle jet orifice; ventilating air outlet 16 through which the heated Ventilating air passes by way of suitable conveying ducts to the space to be heated; an outlet air temperature sensing element 18 disposed in the path of heated Ventilating air being discharged from the outlet 16; a heater air inlet 20 Vinto which `air returned from the conditioned space is Yp.s.i. (pounds per square inch). The excess fuel to valve 32 is bypassed from the regulating valve through a secondary bypass line 36 connected to a common tank return line 38. It will be lapparent then that fuel only flows into the nozzle supply line 34 at the mentioned pressure; and if the pressure on the supply side of the regulating valve is below that amount, the regulating valve will remain closed. .Y

When the pump 24 is operating, the position of twoposition (open-closed) valve y40 in primary bypass conduit 42 determines whether the regulating valve 32 will be open or closed. This valve 40 is biased to an open position and snaps to a closed position when solenoid 44 is energized. With the pump 24' operating and the valve 40 closed, the pressure in line 30 rapidly builds up and causes the regulating valve 32 to open and pass fuel into the nozzle supply line. When valve 40 opens, the pressure in line l30 very quickly decreases to a value below that required to maintain the regulating valve open, vand 3 all the fuel will bypass back to the common tank return line 38 through the primary bypass line 42.

Under normal operating conditions with valve 40 closed -and regulating valve 32 open, the fuel supplied to nozzle supply line 34 passes to the inlet side of the nozzle 12 where part of it is discharged into the burner 6 and part of it passes from the outlet side of the nozzle into a single nozzle return line 46. This nozzle return line contains a check valve -48 which prevents reverse dow of fuel into the nozzle under any conditions.

The single nozzle return line `46 is divided into a pair of parallel nozzle return lines 50 and 52. Line 50 contains a throttling valve 54 connected by capillary line 6 to the heater outlet air temperature sensing element 18. Throttle valve S4 is also provided with a tempera- -ture selection knob 58 for selecting a desired discharge air temperature. Parallel line 52 contains throttle valve 60 connected by capillary line 62 to inlet air temperature sensing element 22. Throttle valve 60 also has a temperature selection knob 64. The fuel outlet side of each of these valves 54 and 60 is connected to the common tank return line 38.

The throttle valve 54 is adjusted by means of the temperature selection knob 58 to control burner tire at a rate providing a selected discharge air temperature. The sensing element 18 associated with the valve 54 senses the discharge air temperature produced by the heater and causes the valve to move in an opening or closing direction in response to a sensed discharge air temperature respectively above or below the selected discharge Vair temperature. Such a valve is known in the art and further explanation is not considered necessary herein.

Valve v60 is structurally and operationally similar to valve 54 but several differences should be noted. In this connection, the element 22 for valve 60 is mounted in the inlet air passage for the heater and therefore senses return air while element 18 controlling valve 54 senses discharge air. Also, the control element 22 and capillary line are charged to respond and exert a modulating control on valve 60 within a temperature range of, for example, 60 lto 90 iF., whereas the element 18 for valve S4 is charged to respond in a temperature range, for example, of 130 to 180 F. If desired, valve 60 may be provided with a port sized to permit -a lower back fuel back-pressure at maximum open position than valve 54.

It will be apparent that either valve can affect back pressure in the nozzle return line and consequently exert some control over the rate at which fuel is discharged from the nozzle into the burner. If both valves are in a closed position, the mwimum quantity of fuel will be discharged into the burner. With valve 60 closed and valve S4 open to its maximum position the fuel rate into the -burner will be substantially reduced. If valve 60 has a ylarger' maximum port opening than valve S4, then with valve 54 closed and valve 60 open to its maximum position (corresponding to the minimum heating capacity with continuing burner re) the fuel rate into the burner will be approximately the minimum value at which a stable flame can be supported. When this occurs the fuel 'back-pressure in the nozzle return line will be at its practical minimum since the maximum quantity of fuel will be bypassed through the nozzle and through the valve 60 back to the common tank return line. This will also -be indicative that under the conditions then existing the space is being overheated.

To the end of avoiding this overheating, and in accordance with this invention, means are provided to terminate all fuel dow to the burner by bypassing it around the nozzle burner. Thus, a nozzle shunt conduit or line 66 has one end connected to a diverting or shunt valve 68 in the nozzle supply line 34 and has the other end connected to the nozzle return line 46 at a point between the check valve 48 and the parallel throttle valves 54 and 60. The shunt valve 68 (which is shown in FIGURE l in a position preventing fuel ow into the nozzle supply line 34 and permittinghow into the nozzle shunt conduit 66) is controlled by a solenoid 70. The solenoid is energized when electrical switch 72 in electrical line 74 is closed. Closure of switch 72 is controlled .by fuel pressure responsive means 76 connected to sense fuel pressure in a fuel line in open communicatoin with andl containing lfuel at the same pressure as the nozzle return line. As will be readily appreciated, the fuel pressure responsive element is biased in a direction opposing the force exerted by the fuel pressure so that switch 72 is closed when the fuel pressure sensed is -below a predetermined minimum. The element 76 can be set, for example, to close switch 72 in response to a fall in pressure below 24 p.s.i. and to open switch 72 in response to a rise in pressure above 32 p.s.i.

Assuming the shunt valve -68 is in a position passing fuel to the nozzle, when the fuel pressure in the nozzle shunt conduit 66 (which is connected to the nozzle return line and therefore contains fuel at the same pressure as in the nozzle return line) falls to the predetermined minimum indicating valve 60 is open to its practical limit with respect to a stable flame, the fuel pressure responsive means 76 causes switch 72 to close. This energizes solenoid 70 which actuates the shunt valve 6 8 to a position diverting fuel flow from the nozzle supply line 34. into the nozzle shunt conduit 66. This cuts oft' burner tire since fuel is then not available at the nozzle for discharge into the burner.

To avoid any abrupt change in the fuel pressure sensed by the element 76 when fuel is diverted, an adjustable restricting orifice 78 is provided in the shunt conduit. This orifice is adjusted to provide the same resistance to fuel owas the nozzle 12, check valve `48 and connecting lines 34 and 46.

After valve 68 is switched to its shunt position, fuel will continue to ow in the nozzle shunt conduit 66 until the fuel back-pressure sensed by lfuel pressure responsive element 76 rises above the predetermined low value` Therrise in fuel back-pressure sensed by the element'76 will b e occasioned by valve 60 moving towards a closed position from its wide-open position. This closing movement of valve 60 in turn results from inlet air temperature sensing element 22 detecting that the inlet air temperature has dropped due to the burner re being cut off. When the sensed fuel pressure in the nozzle shunt oo nduit rises, switch 72 will be opened by element 76, the Solenoid 70 -de-energized, and the shunt valve will return to a position restoring fuel flow to the nozzle. Thus, burner fire resumes at a relatively low level.

If continued burner tire at this low level again causes valve 60 to assume -a wide-open position, the correspondingly decreased -fuel back-pressure will again result inactuation of the shunt valve to a position terminating fuel ow tothenozzle. Thus the on-otf burner fire will continue so long as the bur-ner re rate corresponding to a near minimum fuel back-.pressure results in substantial overheating of the space as correspondingly sensed 4by the return air element 22.

The fuel pump 24 is driven by 'accessory drive motor 80 which `also drives the ignition magneto 82 connected by electrical line 84 to electrodes 14. When an electrical power source is available to power certain of the components associated with the heater, a circuit arrangement such las shown in FIGURE 2 may be provided. Closure` of line switch 86energizes the ventilation air motor 2, cornbustion air motor 4, and an accessory drive motor 80. With these elements operating, the heater is in a condition for combustion to be established. That is, the required air flow to support combustion, and to receive the heat released by the combustion is available, and the fuelis being pumped through the prim-ary bypass line 42 back to the tank 28.

A manually closable combustion starting switch 88. is arranged in series with: a holding relay coil 90 having solenoid 44 which controls the main burner fuelvalve 40 in parallel Itherewith; a normally closed overheat switch 92 which may be conveniently located to sense an overheated heater condition and open in response thereto; and an air ow responsive switch 94. When energized, coil 90 operates to close its asociated switch 96 so that the manually closed switch 88 may be released while the switch 96 maintains the circuit complete.

It will be understood that when the heater is being operated `from a cold start, both valves 54 and 60 will be closed because of the demand for heated air at both the outlet and inlet of the heater. Also, shunt valve 68 will be in a position to divert fuel ow into the shunt conduit since, until regulating valve 32 opens and permits fuel ow via one Way or another to the throttle valves, the fuel back-pressure sensed by element 76 is below the minimum value.

Now, upon closing the combustion starting switch 88 the solenoid 44 is energized and closes burner fuel valve 40. This causes the fuel pressure which builds up in line 30 to open regulating valve 32. Fuel will then ow through shunt valve 68 into the shunt conduit 66 to the closed throttle valves 60 and 54. Thus the fuel backpressure quickly rises to a value which causes pressure responsive element 76 to open electrical switch 72 in line 74. This de-energizes the solenoid 70 controlling shunt valve 68 and valve 68 is actuated to a position permitting fuel ow to the nozzle 12. Since the throttle valves are fully closed, all of this fuel is discharged into the burner. Since an igniting spark is available at xall times that fuel pump 24 is operating, combustion will be initiated in the burner as soon as fuel is discharged from the nozzle.

The heated Ventilating air temperature will soon rise to the selected temperature and warm the condition space. As explained heretofore, after the discharge air temperature rises to its selected value, the throttle valve 54 will operate in an attempt to maintain the discharge air temperature at the selected value. Howe-ver, assuming the temperature of the inlet air sensed by element 22 ultimately rises above its selected value, the throttle valve 60 will open in response thereto and assume control of fuel backpressure while throttle valve 54 will move to a closed position.

Then, if the heater inlet air temperature rises to a value causing throttle value 60 -to move to a fully open position, this condition will, in accordance with previously described operation, cause the shunt valve 68 to divert fuel away from the nozzle and thus cut off burner fire. With the burner tire cut olf, the conditioned space temperature will fall until the heater inlet air temperature sensed by element 22 causes valve 60 to move towards a sufficiently closed position to build up fuel back-pressure to a value operable to switch fuel ow back to the nozzle and re-establish burner tire. Thus the heater willoperate with either 'modulating fuel ow to the burner or i-n an on-off fashion in accordance with the heating demands manifested at the inlet to the heater.

FIGURE 3 illustrates a portion of an alternate fuel system having a shunt valve actuated Ior powered by fuel pressure rather than being electrically powered. Such an arrangement is desirable for heaters which are selfpowered in the sense that no outside electrical power is used. In that connection and as disclosed in the noted Hubbard patent, an internal combustion engine prime mover is used to drive a combined ventilation air-combustion air blower, a gear box take-off is used for the accessory drive, and the main bypass burner valve is arranged to be manually closed and maintained in a closed vposition by fuel pressure.

As shown in FIGURE 3, the fuel line 102 leads from the fuel pressure regulating valve (not shown) to an inlet port 104 of shunt valve dignated ygenerally 106. One outlet port 108 is connected to the nozzle supply line 110, while the alternate outlet port 112 is connected to nozzle shunt line y114. It is noted that nozzle shunt line 114 has 6 an Iadjustable: orifice 11S to permit equalizing resistance to owfin line 114 to that found in the alternate ow path.

The inlet and outlet ports are connected to a common chamber 116having slide valve 118 therein. The end portions of slide valve 118 are of substantially the same cross-section las the chamber while the central portion of the slide valve is of reduced cross-section for a length sufficient to provide communication between the inlet port 104 and either of the alternate outlet ports 108 or 112. Each end of the chamber is connected by a fuelrelief bore to a common fuel relief bore 120 which in turn terminates in a -fuel relief port 122. This arrangement permits -any fuel which accumulates in the ends of the chamber to be passed to a fuel relief conduit 124 when the slide valve is displaced in either direction.

The slide valve 118 is actuated by a drive rod 126 secured at one end to the slide valve and sealably passing through left end -Wall of cylinder 128. Cylinder 128 contains piston 130 which is biased to the left by balance spring 132. The piston slides freely, within limits, upon drive rod k126 which extends through the center o-f the piston face and has secured thereto on oposite sides of the face boss-like piston stops 134 and 136. The left-hand chamber of the cylinder contains several overcenter snap springs 138 directed radially inwardly from secured position in the cylinder wall and which function to give bias to the piston in either direction when the centrally disposed, movable ends of the springs move to either side of the neutral axis'of the springs. The right-hand charnber of the cylinder includes a fuel relief port 140 also connected to relief line 124.

The left-hand chamber of the cylinder receives fuel through port 142 connected by line 144 to the nozzle return line 145 at a point between the downstream side of the check valve 147 and the upstream side of throttle valves 146 and 148. Thus, an increase in fuel pressure in the nozzle return line causes `a corresponding increase in fuel pressure in the left-hand chamber and tends lto move piston 130 tothe right against balance spring 132. A decrease in fuel pressure in the nozzle return line and in the left-hand chamber tends to permit the spring 132 to move the piston to the left. The tension of the balance spring 132 can be increased or decreased by adjusting knob 150.

With the elements in the positions shown in FIGURE 3 (which lare the positions they would be in when the heater is started) and assuming the pressure regulating valve opens to pass fuel into line 102 and that both throttle valves 146 and 148 are closed to demand heat, then, fuel pressure Iwill rapidly build up in the nozzle return line and consequently also build up in the lefthand chamber of the cylinder. 'Ihis build up of pressure causes the piston to progressively move to the right upon rod 126. When the piston engages the right-hand stop 136 and causes movement of the stop and rod to the right, the centrally disposed ends of the snap springs 138 will be moved to the right past dead center and will thereby contribute to the force to the right causing rod 126 and slide valve 118 to plunge -to an extreme right position. This movement closes port 112 and opens port 108 so that fuel ow is switched from the nozzle shunt line 114 to the nozzle supply line 110. With fuel supplied to the nozzle and discharged therefrom, burner iire is initiated.

Now assume that in accordance with previous explanation the heating demands become satisfied and the con- ,ditioned space begins to overheat so that throttle valve 148 progressively moves to an extreme open position. As this occurs, the fuel pressure in the nozzle return `line 145 and in the left-hand chamber of the cylinder 128 is correspondingly progressively reduced. As the pressure is reduced, it reaches a value at which the leftwardly directed force of the balance spring 132 overcomes the n'ghtwardly directed combined force of the fuel presssure lplus overcenter snap springs 138. As the overcenter springs are moved to the left of'. a deadce'nter: position the'drive r'od 126 and slide valve 118 will be. plunged to' the left. This will close shuntfvalve port 108' and'open port 112 so thatV fuel ow will be switched to the'nozzle shunt conduit. Upon a subsequent movement ofthrottle valve 148 away from a fully openl position inY response tol a demand for more heat, the nozzle return line fuel pressure and left-hand chamber fuel pressure Will risc causing the slide valve 118 to again be moved to itsalternate position.

The invention is considered to have its prim-ary app1ication in a substantially closed air system having-separately responsive throttle valves, and wherein variations ingheater inlet air temperature reflecting a space temperature cause variations in burner tire. However, the inventive concept is also applicable to an` airheater which has, in place of the parallel throttle valves, a single throttle valve controlled by discharge air temperature. For such service this single throttle valve is designed so that in a wide open position in which it exerts` a' minimum resistance to fuel flow the fuel back-pressure s at a low value corresponding to the lowest rate ofv'fuel discharge into the burner which will support a minimum stable lire. rlfhis minimum fuel back pressure will cause fuel to be shunted around the nozzle and stop burner re as 'hereto fore'disclosed until the discharge air temperature drops sufficiently to cause the discharge temperature sensing element to move the throttle valve in a closing direction to increase fuel back-pressure above the minimum and restore burner fire. It will -be appreciatedl that under normal conditions, the change from modulating -to on-off control of the burner will onlyv occur when the minimum heating capacity causesthe'tempei'ature of the discharge air to exceed the settingsufiiciently to cause the throttle valve to move to a wide open position. Suche condition arises when a heater having either a relatively low air flow volume, or a high minimumheating capacity, or both, is used with incoming air of a temperature suiciently high to cause such acondition.

Having described my invention, I claim:

l. A fuel and'combustion control systemvfor an air heater comprising: a fuel pump; a` fuel pressure regulating valve connected to receive fuel from said pump, and pass it at a relatively constant pressure; a nozzle supply conduit; Ia. diver-ting valve connected rto receive said fuel, said valve being operable to a rst position fork passing said fuel to said nozzlesupply conduit, and to a second position diverting fuel from said nozzle supply condu-it; a bypass type nozzle having an inlet connected to said nozzle supply conduit, an orifice for discharging fuel into a burner, and a bypassed fuel outlet; a nozzle return conduit connected to said nozzle outlet; a check valve in said return conduit 'to prevent reverse flow of fuel therein; first and second branch return conduits yconnected to said nozzle return conduit; a first throttle valve in said rst branch conduit operable towards open and closed positions in response to discharge air temperatures produced by said burner respectively above and below a rst selected air temperature Within a rst air temperature range; a second throttle valve rin said Vsecond branch conduit operable towards'open and closed positions in response to air temperaturesof inlet air to be heated by said burner respectively above and belo'w a` selected air temperature Within la second relatively lower air temperature range; a nozzle shunt conduit connected to said divertingY valve for receiving fuel when said diverting valve is insaid second position and passing said diverted fuel to said nozzle return line rat a point between said check valve and said throttle valves; means responsive to the-fuel back pressure at saidV point for operating said diverting valve to said second'positon in response to a fall in fuel back -pressurecorres'pondingvto said second throttlepvalve movingto a `fully open. position, and to said rstiposition in response to a rise in b fuel back pressure corresponding to saidA second throttle valvefmovingfrom said fully open position.

2. The system of claim l including: main fuel bypass means including a main bypass conduit havingone end in communication with the outlet of said fuel pump and having a two position valve therein adapted to be maintained in a closed position `during normal heater operation and operable to an open position in response to a predetermined maximum air temperature indicative of said burner producing overheated air.

3. The system of claim l including: means in said nozzle shunt conduit for adjusting the resistance to fuel flow through said shunt conduit to substantially match the resistance to fuel flow encountered in the alternate ilow path including said nozzle and said check valve.

4. In a fuel and combustion control system for an air heater adapted to be lconnected normally to receive air to be heated from the space being conditioned by said heater: means for supplying fuelr at aV relatively constant pressure; a diverting valve connected to receive said fuel and hav ing a rst outlet and an alternate outletya nozzle return line; a bypass type nozzle connected to receive fuel from said first outlet and adapted to discharge fuel into a burner at a rate dependent upon fuel back pressure in said nozzle return line; a check valve in said nozzle return line; a nozzle shunt conduit connected to said alternate outlet and providing a parallel fuel path around said nozzle to said nozzle return line; throttle valve means in said nozzle return line for varying the fuel back pressure; means for controlling said throttle valve means to vary fuel back pressure in response to departures in air temperature produced by said burner from selected air temperatures; and means responsive to a decrease in fuel back pressure to a predetermined minimum corresponding to a practical minimum rate of said fuel discharge into said burner to actuate said diverting valve to said alternate outlet position so that fuel flow to said nozzle is termina-ted.

5. In -a fuel and combustion control system for an air heater: a nozzle fuel supply conduit; a nozzle fuel return conduit; a bypass type burner nozzle connected between said supply and return conduits and adapted to receive fuel and to discharge fuel into a burner at a rate dependent upon fuel back pressure in said Ireturn conduit; means for supplying fuel to said nozzle fuel supply conduit at a relatively constant pressure; a check valve in said nozzle return conduit preventing reverse fuel dow in said return conduit; throttle valve means in said returnr conduit for varying said fuel back pressure in response to departures in the temperature of the heated air produced by said burner from a selected temperature; a fuei bypass conduit connected between said nozzle supply conduit and said nozzle return conduit for shunting fuel around said nozzle; a shunt valve operable in a first position to shunt fuel into said bypass conduit and around said nozzle, and a second position to pass fuel to said nozzle and shut-off said bypassconduit; and means responsive to a fall in said fuel back pressure to a predetermined minimum value to operate said shunt valve means to said first position and thereby terminate fuel supply to said nozzle, `and responsive to said fue-l back pressure being above said minimum predetermined value to operate said shunt valve means to said second position and restore fuel supply to said nozzle.

6. A fuel and combustion control system for an'air heater, comprising: a source of liquid fuel; fuel pump means; a fuel pressure regulating valve connected to receive fuel from said pump means and operable, when open, to pass it at a relatively constant pressure; a main bypass line having one end connected between said pump means and said regulating valve, said line including a valve therein operable, when open, to permit the return of fuel to said fuel source and operable, when closed, to cause said regulating valve to open and pass fuel; means to close said main bypass line valve when normal heater operation is to be initiated; a fuel lineincluding a shunt valve connected to receive fuel from said regulating valve, said shunt valve being operable to alternate positions; a nozzle supply line connected to receive fuel from said shunt valve in the first of said alternate positions; a nozzle return line; a burner nozzle of the bypass type connected between said nozzle supply line and said nozzle return line, said nozzle return line including a check valve to prevent reverse flow of fuel therein; a nozzle shunt line connected to receive fuel from said shunt valve in the second of said alternate positions and shunt said fuel around said nozzle to said nozzle return line; throttle valve means connected downstream from said check valve and operable to increase and decrease fuel back-pressure in said nozzle return line in response to departures in air temperature produced by said burner respectively below and above a selected temperature; `and fuel pressure responsive means responsive to a fall in fuel pressure in said nozzle return line below a predetermined low range to operate said shunt valve to said second alternate position for terminating `all fuel flow to said nozzle by shunting said fuel around said nozzle, and to a rise in fuel pressure in said nozzle return line above said low range to operate said shunt valve to said rst alternate position for restoring fuel flow to said nozzle.

7. The system of claim 6 wherein: said fuel pressure responsive means includes a piston cylinder having a chamber in communication -with said nozzle return line so that said chamber is adapted to contain fuel at substantially the same pressure as said nozzle return line; piston means in said chamber movable within limits in one direction upon an increase in said fuel pressure and in an opposite direction upon a decrease in fuel pressure; and means connecting said shunt valve for actuation to said alternate positions in accordance with movement of said piston means.

8. The system of claim 7 including: means biasing said piston means in said opposite direction throughout the range of said piston means travel; and separate means biasing said piston towards either end travel position from a predetermined intermediate neutral position of said separate biasing means, so that in a movement of said piston means from one extreme end travel position to another said separate biasing means provides force resisting said movement until sa-id piston means causes said separate biasing means to pass through neutral position and then provides force contributing to said piston means movement.

References Cited in the le of this patent UNITED STATES PATENTS 2,262,825 Welliver Nov. 18, 1941 2,290,350 Olches July 21, 1942 2,334,679 Mason et al Nov. 16, 1943 2,794,599 Irwin June 4, 1957 

